1. Field of the Invention
The present invention relates to a magnetic head driving device for generating a modulated magnetic field, adapted for use in a magnetooptical recording apparatus.
2. Related Background Art
The magnetooptical recording/reproducing apparatus is expected to be a large-capacity auxiliary memory of the next generation, and is being commercialized. There is a strong demand toward a higher transfer rate, and various technologies are being researched and developed for meeting such a requirement. Among such technologies there is known the magnetic field modulation overwriting technology, which is to record information by applying a magnetic field, modulated according to the recording data, to the recording medium, simultaneously with irradiation of a laser beam focused onto the recording medium. In contrast to the magnetooptical recording apparatus of the light modulation process, in which the information recording operation requires two processes of (1) erasure of old data, and (2) writing of new data, the magnetic field modulation overwriting technology can complete the above-mentioned two processes in one process, thereby allowing an increase in the transfer rate at the recording operation.
However, the magnetic head to be employed in the magnetooptical recording apparatus utilizing the magnetic field modulation overwriting technology is different from that of the fixed magnetic field recording apparatus in that it has a large inductance and it cannot generate the required magnetic field unless it is given a large current, because of the following reasons. Firstly, it is required that a perpendicular magnetic field be applied instead of a longitudinal (in-plane) magnetic field, and a closed magnetic circuit cannot be formed because of the presence of an optical head across the recording medium. Secondly, the area of magnetic field application has to be made wider, in order to facilitate the adjustment of positions of the magnetic field application and of the laser beam spot. Thirdly, the distance between the magnetic head and the recording medium has to be increased, for the purpose of protection thereof. A magnetic head driving circuit, for providing a magnetic head of a large inductance with a large current for achieving the above-mentioned requirements is disclosed in the Japanese Patent Laid-Open Application No. 63-94406.
FIG. 1 is a circuit diagram showing the above-mentioned magnetic head driving circuit, in which shown are a magnetic head 1 and auxiliary coils L1, L2. The magnetic head 1 is composed, in practice, of an unrepresented core and a coil L wound thereon. Switching elements T1, T2 are required to switch currents of 100 mA or larger within an extremely short time, and are generally composed of field effect transistors for meeting such a requirement. In the following description, the field effect transistors T1, T2 are simply called transistors. Driving circuits 2, 3 are provided for driving gates of the transistors T1, T2. The auxiliary coils L1, L2 are constantly given currents regardless of the turn-on or turn-off state of the transistors T1, T2, and serve to invert the current supplied to the magnetic head 1 at a high speed.
FIGS. 2(a) through 2(g) are timing charts showing signal wave forms in various parts of the magnetic head driving circuit mentioned above, wherein FIG. 2(a) indicates an information signal to be recorded; FIG. 2(b) indicates a drive signal S1 supplied to the drive circuit 2; and FIG. 2(c) indicates a drive signal S2 supplied to the drive circuit 3. The drive signal S1 is the same in phase as the information signal, while the drive signal S2 is inverted in phase from the information signal. In the case of information recording, or, of information overwriting on the previously recorded information, the above-mentioned drive signals S1, S2 are respectively supplied to the drive circuits 2, 3 which generate driving voltages, the same in phase as the drive signals, for supply to the gates of the transistors T1, T2, whereby said transistors T1, T2 are alternately turned on to supply the magnetic head 1 with an AC current .+-.I.sub.LC as shown in FIG. 2(f). In this state, the currents I.sub.T1, I.sub.T2 of the transistors T1, T2 are respectively double that of the magnetic head current I.sub.LC, as shown in FIGS. 2(d) and 2(e). Thus the magnetic head 1 generates a magnetic field .+-.H.sub.B modulated according to the information signal, as indicated in FIG. 2(g). In information recording, the modulated magnetic field is applied by the magnetic head to the magnetooptical recording medium while it is irradiated with a light beam having a constant intensity by the optical head, whereby magnetizations corresponding to the information signal are recorded as information pits on the recording medium.
In contrast to the information recording by magnetic field modulation explained above, the magnetooptical recording medium of the preceding first generation employed the information recording by the optical modulation process, which effects the recording of new information after the previously recorded information is erased, and in which the function of the magnetic head is different from that in the magnetic field modulation process. The principle of the optical modulation process is illustrated in FIGS. 3(a) through 3(d), in which FIG. 3(a) shows the operation mode; and FIG. 3(b) indicates an information signal to be recorded. In the case of information recording, an area to be recorded is at first subjected to the application of a magnetic field -H.sub.B with the irradiation of a laser beam of an erasing power, as indicated in FIGS. 3(c) and 3(d), whereby the old information recorded on the magnetooptical recording medium is erased. After the erasure, the magnetic head applies a magnetic field +H.sub.B, as shown in FIG. 3(c), which has a polarity inverse to that of the magnetic field at the erasure, while the optical head effects irradiation of a laser beam modulated in intensity by the information of a laser beam modulated in intensity by the information signal as shown in FIG. 3(d), whereby the information is recorded on the recording medium.
There will be explained, with reference to FIGS. 2(a) through 2(g), the operations in the case of information recording by optical modulation, employing the magnetic head driving circuit shown in FIG. 1. When the magnetic head is DC driven instead of high frequency drive in the circuit shown in FIG. 1, the transistor T1 is turned off while the transistor T2 is continuously turned on during the erasing period of the optical modulation process. In this case, the auxiliary coils L1, L2 do not work as inductance elements but work as resistance elements corresponding to the DC resistances of the coils. The DC resistances of the auxiliary coils L1, L2 are for example about 0.1.OMEGA., while that of the magnetic head 1 is in the order of 1.OMEGA.. During the erasing period, the transistor T2 receives a current supplied from the auxiliary coil L1 through the magnetic head 1 and a current supplied from the auxiliary coil L2. Consequently, in the erasing mode, the current in the magnetic head 1 gradually decreases, from -I.sub.LC, according to a time constant determined by the inductance and the resistance in the driving circuit, as shown in FIG. 2(f). The current is mostly supplied from the auxiliary coil L2 because of its small resistance, and the current in the magnetic head 1 is determined by energy accumulated in the auxiliary coil L1, the energy being reduced with the above-mentioned time constant. The time constant .tau., with which the current decreases from -I.sub.LC, is given by L.sub.t /R.sub.T wherein L.sub.t is the sum of the inductances of the auxiliary coil L1 and the magnetic head, while R.sub.T is the sum of the resistances of the auxiliary coil L1 and the magnetic head and the on-resistance of the transistor T2. R.sub.T is mostly determined by the on-resistance of the transistor T2, which is sufficiently larger than the resistances of the auxiliary coil and the magnetic head.
For example, the on-resistance is about 25.OMEGA., for a power source voltage of 5 V and I.sub.LC of 200 mA. When inductances of the auxiliary coil L1 and the magnetic head are 100 .mu.H and 1.5 .mu.H, respectively, the time constant .tau. becomes about 4.06 .mu.S. Thus, during the erasing period, the magnetic head current gradually decreases with the above-mentioned time constant, as shown in FIG. 2(f), eventually to a value determined by the ratio of the resistance of the auxiliary coil L1 and that of the magnetic head and the auxiliary coil L2. For example, an initial magnetic head current I.sub.LC of 200 mA decreases to 18.2 mA. Therefore, the magnetic field generated by the magnetic head, being proportional to the current, gradually decreases from -H.sub.B according to the time constant .tau. as shown in FIG. 2(g), and the old information cannot be erased because of the varying magnetic field. Also, in the recording operation, the magnetic head current decreases as indicated in FIGS. 2(f) and 2(g), and the generated magnetic field becomes deficient for the information recording. The magnetic head driving circuit of the magnetic field modulation process is thus incapable of information erasure or recording in the optical modulation process, so that the magnetooptical recording apparatus has to be provided with exclusive driving circuits for achieving the recording by the magnetic field modulation process and by the optical modulation process.
Also, in the magnetic head driving circuit shown in FIG. 1, the potential at a point a or b when the transistor T1 or T2 is turned on is given by the power source voltage minus the voltage drop in the auxiliary coil. More specifically, for an amplitude .+-.I.sub.LC of the AC current in the magnetic head, the potential V.sub.0 at the point a or b is given by V-R.sub.L .multidot.T.sub.LC wherein V is the power source voltage and R.sub.L is the DC resistance of the auxiliary coil. In FIG. 4 there are shown the currents in the transistors T1, T2 and the potential V.sub.0 at the points a, b. Also, when the transistor T1 or T2 is turned on, there will flow a current 2I.sub.LC, so that the transistor T1 or T2 consumes an average power of V.sub.0 .times.2I.sub.LC /2 even when the recording data do not contain the DC component. The DC resistance of the auxiliary coils is generally small and can be assumed to be on the order of 0.5.OMEGA.. Also, assuming that the DC power source voltage is 5 V and the modulating current amplitude of the magnetic head is 300 mA, V.sub.0 becomes 4.85 V and the average power consumption becomes about 1.455 W when the recording data are free from the DC component.
When a high-speed power MOS FET is employed as the switching element there will result a breakage of the device by the power consumption mentioned above, because the maximum nominal power rating of such a device is about 1 W. Also, current drive of the magnetic head is conducted by providing the power source with a constant current source as shown in FIG. 1, and, in such a case, the transistor is not necessarily overloaded because the aforementioned power is consumed by the transistor and by the constant current source. However, since such a constant current source is generally composed of bipolar transistors or the like, there may still be encountered the destruction of the components of such a constant current source if the power consumption thereof is large. It is also conceivable to employ a switching element of a large nominal power rating, but a high transfer rate cannot be realized because such a switching element of a high power rating is inevitably associated with a slower switching speed. On the other hand, when a high-speed switching element is employed, it is necessary to conduct heat dissipation appropriately in order to prevent destruction of the element.
Also, in the magnetooptical recording apparatus employing the magnetic field modulation overwriting technology, a large current has to be supplied to the magnetic head having a large inductance, and, in order to achieve a high transfer rate, it is necessary to reduce the time period for inversion of the magnetic field, thereby increasing the frequency of repetition. For recording data of a repeating frequency of several MHz, the time period for magnetic field inversion generally has to be shorter than 20 ns. However, because the magnetic head has a large inductance, the time period of magnetic field inversion cannot be made short if the circuit connected to both ends of the magnetic head has a large floating capacitance. The time period for magnetic field inversion cannot be made shorter than a minimum value determined by the inductance of the magnetic head and the floating capacitance of the circuit connected to both ends of the magnetic head, regardless of the structure of the circuit. In the magnetic head driving circuit shown in FIG. 1, the time period for magnetic field inversion is about 100 ns if the magnetic head has an inductance of 1 .mu.H and the floating capacitance is about 1 nF.
Also, a short magnetic field inversion time period results in an increased loss in the magnetic head. The loss in the magnetic head at a high frequency can be considered, in an equivalent circuit consisting of a parallel connection of an inductance L.sub.p and a resistance R.sub.p, as the loss by a current in the resistance R.sub.p, and, when the magnetic field inversion time period is shortened, the maximum frequency in the recording frequency range becomes higher, whereby the current in the resistance R.sub.p increases to enhance the loss in the magnetic head. For this reason the magnetic head shows considerable temperature increase, and the core of the magnetic head may eventually exceed the Curie point, whereby the magnetic permeability of the magnetic material may become smaller and the generated magnetic field may become insufficient for information recording.
FIG. 5 shows the above-mentioned current in the resistance R.sub.p. For example, a magnetic head with an inductance of about 1 .mu.H and with R.sub.p of about 200.OMEGA. is used in modulated drive with a magnetic field inversion time period of about 10 ns and a repeating frequency of ca. 6 MHz, the loss in the resistance R.sub.p becomes about 0.5 W. However, the loss in the magnetic head has to be about 0.2 W or less, in consideration of the temperature increase of the magnetic head itself. In this manner the magnetic field inversion time period cannot be made shorter if a large floating capacitance is connected to the ends of the magnetic head, while a shorter magnetic field inversion time period increases the loss in the magnetic head, thereby disabling the information recording.
Also, in the conventional example explained above, the voltage supplied to the gates of the transistors cannot be made higher than the power source voltage employed in the apparatus. A recent trend is to reduce the power source voltage in the apparatus in order to decrease the power consumption, but such reduced power source voltage also reduces the voltage supplied to the gate of the transistor, thereby decreasing the amplitude of the modulated current. FIG. 6 is a timing chart of the conventional magnetic head driving circuit, in which the high-state voltages released from the driving circuits 2, 3 are determined by the power source employed in the apparatus. Thus, a reduced power source voltage lowers the output voltages of the driving circuits 2, 3, thereby decreasing the voltage supplied to the gates of the switching transistors, whereby the currents therein are also decreased to reduce the amplitude of the modulated current in the magnetic head, thereby resulting in a deficiency in the generated magnetic field.
Furthermore, in the above-explained conventional magnetic head driving circuit, the off-time of the switching elements has to be shortened in order to decrease the magnetic field inversion time period, but this is in fact difficult to achieve because the FET employed as the switching element has a long off-time. Furthermore, if the on/off timings of the switching elements by the driving circuits 2, 3 are somewhat shifted, an inverse voltage is generated across the magnetic head, thereby generating an inverse current in the switching elements and distorting the waveform of the modulated current.